Abstract Details
(2020) A Two Stage P-T-T History of the Sanbagawa Metamorphic Terrane Constrained by Grossular U–Pb Geochronology
Niki S, Yoshida K, Sawada H, Oyanagi R & Hirata T
https://doi.org/10.46427/gold2020.1932
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06n: Room 2, Wednesday 24th June 23:48 - 23:51
Sota Niki
View all 2 abstracts at Goldschmidt2020
View abstracts at 4 conferences in series
Kenta Yoshida View all 2 abstracts at Goldschmidt2020 View abstracts at 8 conferences in series
Hikaru Sawada View abstracts at 5 conferences in series
Ryosuke Oyanagi View all 3 abstracts at Goldschmidt2020 View abstracts at 3 conferences in series
Takafumi Hirata View all 4 abstracts at Goldschmidt2020
Kenta Yoshida View all 2 abstracts at Goldschmidt2020 View abstracts at 8 conferences in series
Hikaru Sawada View abstracts at 5 conferences in series
Ryosuke Oyanagi View all 3 abstracts at Goldschmidt2020 View abstracts at 3 conferences in series
Takafumi Hirata View all 4 abstracts at Goldschmidt2020
Listed below are questions that have been submitted by the community that the author will try and cover in their presentation. To submit a question, ensure you are signed in to the website. Authors or session conveners approve questions before they are displayed here.
Submitted by Taehwan Kim on Wednesday 24th June 13:17
Thank you for introducing grossular dating technique. I just wonder that the MSWDs (i.e. 3-4) for mean ages of core and rim are much higher than unity, so you may need further careful "discrimination" for the ages. Anyway, low-U garnet dating is impressive! Taehwan
Thank you for your attention and advice. Actually, we tried to distinguish the data-points but failed. In this study, we distinguish core and rim domains based on the andradite content. Now I'm thinking about using other chemical signatures like REEs to distinguish the data-points. The potential causes of the large variation of data-points with large MSWDs are ?the heterogeneity of initial lead isotope ratios during crystallization, ?mixing of core and rim domains in depth direction due to the relatively large depth of ablation pits (c.a. 30 µm), and ?mineral inclusions in ablation pits. Shallower ablation pits of U-Pb dating are also desirable. >Anyway, low-U garnet dating is impressive! I'm glad to receive your comments!
Thank you for introducing grossular dating technique. I just wonder that the MSWDs (i.e. 3-4) for mean ages of core and rim are much higher than unity, so you may need further careful "discrimination" for the ages. Anyway, low-U garnet dating is impressive! Taehwan
Thank you for your attention and advice. Actually, we tried to distinguish the data-points but failed. In this study, we distinguish core and rim domains based on the andradite content. Now I'm thinking about using other chemical signatures like REEs to distinguish the data-points. The potential causes of the large variation of data-points with large MSWDs are ?the heterogeneity of initial lead isotope ratios during crystallization, ?mixing of core and rim domains in depth direction due to the relatively large depth of ablation pits (c.a. 30 µm), and ?mineral inclusions in ablation pits. Shallower ablation pits of U-Pb dating are also desirable. >Anyway, low-U garnet dating is impressive! I'm glad to receive your comments!
Submitted by Yankun Di on Wednesday 24th June 15:03
What is the advantage of your new U-Pb dating of garnet over the existing Lu-Hf dating?
Thank you for your attention. LA-ICP-MS U-Pb dating is an in-situ analytical method and we can carefully select the ablation spots for measurements while observing textures inside mineral samples. The application of Lu-Hf dating to grossular garnet is limited because of the relatively high initially incorporated non-radiogenic Hf compared with other datable garnet like almandine. Moreover, in-situ analytical techniques is not applicable to Lu-Hf dating because of the spectral interference of Yb, Lu, and Hf.
What is the advantage of your new U-Pb dating of garnet over the existing Lu-Hf dating?
Thank you for your attention. LA-ICP-MS U-Pb dating is an in-situ analytical method and we can carefully select the ablation spots for measurements while observing textures inside mineral samples. The application of Lu-Hf dating to grossular garnet is limited because of the relatively high initially incorporated non-radiogenic Hf compared with other datable garnet like almandine. Moreover, in-situ analytical techniques is not applicable to Lu-Hf dating because of the spectral interference of Yb, Lu, and Hf.
Submitted by Yuri Amelin on Wednesday 24th June 21:56
At the current precision level the core and rim ages are not resolved. What changes in the measurement procedure could you suggest to improve precision?
The repetition rate of laser in this study was 300 Hz, but the limitation is up to 60 kHz. By the higher repetition rate, the S/N ratios of lead isotope signal intensities can be improved and we can measure with shallower ablation pits and avoid analyzing mixture of core and rim domains or mineral inclusions.
At the current precision level the core and rim ages are not resolved. What changes in the measurement procedure could you suggest to improve precision?
The repetition rate of laser in this study was 300 Hz, but the limitation is up to 60 kHz. By the higher repetition rate, the S/N ratios of lead isotope signal intensities can be improved and we can measure with shallower ablation pits and avoid analyzing mixture of core and rim domains or mineral inclusions.
Submitted by Chloe Bonamici on Wednesday 24th June 23:08
Did you investigate your ablation pits after analysis to ensure that no inclusions were intersected? With such low U and Pb, even very small inclusions could affect the measured isotope ratios.
Thank you for your kind attention and advice. Your suggestion may be true. The grossular garnet studied in this study have mineral inclusions like carbonate. We carefully observed the surface of the grossular and selected the ablation pits, but we cannot detect the mineral inclusions in depth direction. The wide range of Pb/U ratio on the Concordia diagram can be explained by the mineral inclusions which contain non-radiogenic lead.
Did you investigate your ablation pits after analysis to ensure that no inclusions were intersected? With such low U and Pb, even very small inclusions could affect the measured isotope ratios.
Thank you for your kind attention and advice. Your suggestion may be true. The grossular garnet studied in this study have mineral inclusions like carbonate. We carefully observed the surface of the grossular and selected the ablation pits, but we cannot detect the mineral inclusions in depth direction. The wide range of Pb/U ratio on the Concordia diagram can be explained by the mineral inclusions which contain non-radiogenic lead.
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